The present invention relates to a method of improving the SVI of a mixed liquor in an aeration tank in an activated sludge treatment system.
Sewage and other wastewaters containing organic matter are currently treated by the activated sludge process. Activated sludge treatment systems often experience "bulking", a phenomenon wherein the sludge settles poorly in a sedimentation tank because of a floc of low density. When bulking occurs, the concentration of solids in the sludge being withdrawn from the sedimentation tank is decreased, and the resulting decrease in the solids concentration in an aeration tank causes overloading and makes the aeration treatment difficult. As a further problem, part of the sludge overflows the sedimentation tank to impair the quality of the clarified water.
Conventional methods for dealing with bulking include increasing the volume of aerating air, and providing a water holding tank before the aeration tank for operating it under uniform load. If these methods prove ineffectual, a filamentous fungi sterilizer such as chlorine, copper compounds, ozone or chloroform is added to either the aeration tank or the return sludge; or a flocculant is added to agglomerate the sludge solids.
Bulking is an unpredictable phenomenon and its prevention requires an uninterrupted addition of the sterilizer or flocculant. However, addition of these chemicals for an extended period leads to an increased operating cost. Furthermore, the sterilizer acts not only on the specific filamentous fungi but also on the flora of microorganisms in the treatment system, and hence, the sterilizer added in an excessive amount reduces the activity of the microorganisms and impairs the quality of the clarified water.
In commonly assigned Japanese Patent Public Disclosure No. 184493/82, the present inventors proposed an improve method for preventing bulking without incurring the problems with the conventional techniques. This method comprises feeding a concentration system other than the activated sludge treatment system with at least one of the mixed liquor in an aeration tank, the mixed liquor withdrawn from a line connecting the aeration tank and the sedimentation tank, and the settled sludge, and returning to the aeration tank a concentrated sludge of higher solids content leaving the concentration system and, optionally, separated water of lower solids content also leaving said concentration system. The concentrated sludge returned to the aeration tank is dispersed in that tank, but 12 to 24 hours later, the sludge returns to its initial (unconcentrated) state. Even if the concentrated sludge is returned to the aeration tank after it is concentrated to a higher density with a roll press dehydrator or any other suitable device, the sludge returns to the unconcentrated state in about 36 hours. Therefore, in order to improve the SVI of the mixed liquor in the aeration tank by the method of Japanese Patent Public Disclosure No. 184493/82, at least 5 wt% of the total solids content of that liquor must be concentrated and returned within a short time.